Actomyosin ATPase activity of smooth muscle and non-muscle cells is regulated by at least two calcium-sensitive processes: (a) the interaction of caldesmon with actin filaments and (b) the phosphorylation of myosin light chains by specific kinases. Although much is already known about light chain phosphorylation, there is considerable disagreement on the role of caldesmon, particularly in non-muscle cells. The objectives of this proposal are to characterize the Ca2+-dependent regulation of adrenal medullary contractile proteins and to determine the significance of this regulation in light of the primary activity of medullary cells: catecholamine secretion by exocytosis. Experiments are designed to obtain the following information: (1) the rate of medullary myosin light chain phosphorylation, (2) the effect of phosphorylation on specific steps of the ATPase cycle, (3) the affinity of medullary caldesmon for calmodulin and for actin in the presence of Ca2+/calmodulin, (4) the effect of the actin-caldesmon interaction on ATPase activity, and (5) the relationship between myosin phosphorylation and catecholamine secretion in intact or permeabilized adrenal chromaffin cells. Results of these studies should increase understanding of the control of non-muscle cell motility and will advance progress toward the long-range goal of this research: the identification of specific steps of the secretory pathway in which cytoskeletal and contractile proteins are involved.